The never-ending explosion in computing processing power has led to an even greater explosion in demands on that power, forcing the designers of computer systems to package vast amounts of electronic equipment into relatively small areas. In addition to the cost of acquiring and maintaining the electronic equipment itself, large and complex computer systems, such as server farms and large data centers, carry additional infrastructure costs, including the costs of real-estate and cooling systems. Because these infrastructure costs are often very high, the owners of these large systems are moving towards ever higher packaging densities, placing as much computing power as possible in each square foot of space.
The owners of complex systems such as server farms and data centers achieve high packaging density by packing more computer equipment in both horizontal and vertical directions. By way of example, the 5380 Server system produced by NCR Corporation is delivered with up to 512 computing nodes, each with as many as four CPUs, stacked in a cabinet that is only 77 inches tall by 24.5 inches wide by 40 inches deep.
While packing computer components this densely solves many of the problems facing the owners of such systems, it also creates another set of challenges, in particular the proper cooling of heat-generating components, especially the CPUs, within such systems. In attempting to alleviate these cooling problems, designers of this type of equipment use forced-air cooling techniques that are aimed at moving cold air from a cold-air source to the hot components inside the cabinet. Forced-air cooling comes in two basic forms—front-to-back cooling and bottom-to-top cooling. The form that is most appropriate in any particular hardware environment and configuration is governed by how the pieces of equipment are physically packaged and electrically connected together.
FIG. 1 is a side view of a server rack 100 that uses bottom-to-top cooling to accommodate the particular packaging needs of the components in the rack. As shown here, the rack 100 includes four vertically mounted rows of printed circuit assemblies, or electronic assemblies 1101 . . . N, each having one or more heat-generating components, such as the central processing units (CPUs) 1151 . . . M. Thermal energy dissipated by the CPUs 1151 . . . M and other active and heat-dissipating integrated circuit devices on the electronic assemblies 1101 . . . N is removed from the electronic assemblies 1101 . . . N by a flow 160 of cooling air that moves from the bottom of the rack 100 to the top. As the air flows past these devices, the temperature of the air rises, so that the air at the top of the cabinet is considerably warmer than the air at the bottom. The cooling effect of the air is bolstered by fan trays 1401 . . . K that serve to pull greater volumes of cool air through the rack, thereby increasing air-flow velocity over the CPUs 1151 . . . M and other components on the electronic assemblies 1101 . . . N. Nevertheless, the air passing over the components at the top of the rack 100 is considerably warmer, and thus has less cooling capacity, than the air passing over components at the bottom of the rack 100. With electronic components, particularly CPUs, growing increasingly hotter with normal operation, cooling is quickly becoming the deciding factor in determining the vertical density of a server rack.